In fluorescent lighting, there is a requirement for end-of-life detection which is listed in ANSI C82.11 (Consolidated-2002). The requirement states that for lamps with tube diameter which is equivalent to T5 size or smaller must have a protection method when the lamp's end-of-life symptoms occurred to avoid hazardous conditions.
FIG. 1 shows a prior art ballast configuration with an inductor-capacitor (LC) resonator. A lamp 101 is connected in parallel with the resonance capacitor 102 with an alternating power source 103 and the resonating inductor 104 connected in series. To start the lamp the resonator is activated and a high voltage is established across the capacitor which in turn strikes on the lamp. After the lamp is stroke on the lamp voltage falls to a lower value while it operates in the steady state.
It is required that the ballast shall not impair safety when abnormal and fault conditions happen. Abnormal conditions are classified (European standard) as:    a) lamp not inserted;    b) the lamp does not start because one of the two cathodes are broken;    c) the lamp does not start although the cathodes are intact;    d) the lamp operates, but a single cathode is de-activated or broken (rectifying effect).
It is desired that the ballast shall have appropriate protections against the four scenarios listed above. In scenarios a), b) and c), if the lamp is unable to be stroked on during the startup phase, the ballast may develop a dangerously high voltage arc across the two ends of the lamp holder, which causes an electrical shock hazard to the technician who may try to replace the lamp, therefore protection must then be enabled. Typically, there are two common detection methods:    1) Current Sensing detection    2) Voltage Sensing Detection
A common current sensing detection method utilizes the inverter choke current as a sensing parameter. By placing a sensing resistor connected between the source of the low side driver and the ground, the choke current is monitored throughout the startup phase. If the lamp is not present or it cannot be ignited, high current will continue to flow through the sensing resistor since the LC resonator will operate at its resonant frequency after the preheat phase with infinite lamp resistance.
Voltage sensing detection is similar to current sensing detection in which the lamp voltage is utilized as the sensing parameter. Over-voltage condition will occur if the lamp does not strike on as the ballast runs in its resonant mode.
In scenario d), the lamp may suffer an imbalance of current flow in alternating direction which is commonly known as the rectifying effect, where one end of filament act as the cathode and the other end act as an anode. In particular, a broken or disconnected filament in a running lamp is a typical end-of-life failure where current sensing detection and voltage sensing detection may be unable to protect the lamp as the lamp is still in an operation mode. This may result in overheating the filament at one end which may melt the glass tube. Consequently this may cause an electrical shock hazard and overheating hazard to the user.
In all cases, end-of life detection usually involves a combination of detection of high voltage across the lamp and the choke current. If the lamp is unable to be stroked on, then voltage and current sensing techniques are already enough to confirm the lamp is at its end-of-life status. Yet, if the lamp is able to be stroked on, only the lamp voltage, filaments status or the rectification of the lamp current may show the End-of-life. However, detection of lamp voltage is unreliable in a sense that the lamp voltage is highly changeable with different operating ambient temperature. Moreover, electronic ballast with dimming function nowadays is very common and voltage detection technique may not be adequate as the lamp voltage could change drastically at different dimming levels with a difference of typically 30-40%. The consequence would be the lamp safety is seriously impaired unless every condition is checked.
In U.S. Pat. No. 6,819,063, Arthur Nemirow has provided a method to sense the filament status in a fluorescent lamp. His invention includes a DC flyback converter to drive the filaments and a separate alternating power source to drive the lamp with multiple outputs act as voltage sources across each of the filament. Due to the open circuit flyback effect and cross-regulation feature of the converter, output voltage will experience a sharp increase in voltage if the load tends to an open load, i.e. the filament is broken. A threshold voltage is then sensed and triggers a protection mechanism to inhibit the output of alternating power to the lamp. There is an advantage in sensing the filament resistance with an isolated circuitry because the lamp usually has high AC voltage across it, i.e. at least one end of the filaments will be at high voltage. Sensing parts at high voltage would not be ideal since the control logic is at low-voltages. However, such method has a few drawbacks where an isolated component, such as opto-coupler is required and it is commonly known that its current transform ratio (CTR) deteriorates against time, produces a change in the sensed voltage level unintentionally. High component counts with an integrated switch at primary side of the DC flyback converter contribute extra cost. Moreover, it provides no information on the potential differences of the two filaments where the amount of difference could indicate one type of rectifying effect.
Thus, there is a need for improving End-of-life detection mechanism, particularly in sensing the difference between filaments in a fluorescent lamp and also the rectifying effect, which must be immune from factors such as operating temperature and lamp condition. The new method should be relatively economical, while providing a more complete and reliable protections to End-of-life. The present invention addresses these needs as described herein.